[TenTec] The Eagle inflation costs, etc.

[Dr. Gerald N. Johnson]

On 9/29/2010 12:56 AM, Richards wrote:
> Yep - I thought so !  ;-)
>
> Then, why would anyone want a new radio ? How did this happen ?
>
> OK... I am being sarcastic, but really, all this chatter about
> how good it was in the old days is getting me down. Everyone seems
> to say how good it was in the old days, but their shacks are filled
> with modern gear. I don't get it.

There is no question that receivers have generally improved over the 
years. Though not necessarily in all details. Synthesizers and general 
coverage have made it tougher to get super intermod performance. Mixers 
are better though sometimes a receiver maker doesn't always apply the 
best available techniques particularly in the signal handling of the 
second mixer after the VHF roofing filter. There's gain so signals that 
reach the second mixer are stronger than they were at the first mixer. 
Getting the second mixer to handle those strong signals means more LO 
drive power and a more expensive second mixer. Look at ALL the radios 
that fall apart in dynamic range within a few kHz of the desired signal. 
I'm sure there are trade offs both economical and for RF leaks to cause 
TVI from the receiver.

Filter time responses have improved a lot too. So they don't ring 
continuously from line noise and thunderstorm static. One of the most 
obvious differences in receiver performance between the 75S-3B (and mine 
is much modified for better dynamic range) and Tentecs.

These days, most radio designs, even if not overtly using DSP as a 
feature, have a computer or three just for logic controls (which leads 
to menus) because computer logic is way more versatile than wired logic 
and makes memories handy. Then with synthesizers (with their offshoot, 
increased phase noise) needing digital not analog control with a good 
reference to give the stability needed for digital modes. Even the 
Corsair II uses a computer for the digital display and on board keyer. 
Many put more work in the CPU than the RF parts. Certainly more 
complexity which often leads to a complex user interface.
>
> I suspect there must have been trade-offs and compromises as things
> GOT BETTER OVER TIME. Right ? Otherwise, we should start a company
> that makes the old stuff again. There seems to be a market for it.

But the hand wired radio of yore won't happen, it takes too much skill 
and so costs too much. PC boards maybe would do better, but supplies of 
new tubes are very limited.
>
> And I respect Gerry's comment as coming from an insider, and also
> he MUST be able to identify the compromises and trade-offs that
> were made that would cause us to tolerate lower intermod performance in
> newer rigs.

The S-line was the epitome of transmitter intermod. Accomplished by 
choice of tubes and the application of RF feedback, which I presume 
Collins patented so other makers were slowed in applying it. Warren 
Amfahr, W0WL, designed the PA of the KWM-1 introducing that RF feedback. 
He's told me that he was put on the project because Art Collins noticed 
how clean Warren's home brew HF SSB mobile rig was. He told me that the 
6146 was good but discussion with RCA, the main maker, caused RCA to 
change the design for better linearity resulting in the 6146A. I'm 
unable to corroborate that story because Collins rig manuals specify the 
6146 by the same part number over the years though the 6146 was 
continuously available as plain, A, B, and W models and the B and later 
W required a change in the PA neutralization circuit.

Still the production 32S transmitters produced 3rd order 35 dB down, 5th 
order probably 45 or 50 and 7th not detectable.

The ham industry wasn't so bothered by transmitter intermod and TV sweep 
tubes, especially those made for color TV sets were cheaper and worked. 
The venerable 6BG6G looked just like an 807 or 1625 inside but with an 
octal base and less shielding. One day during some boring lecture I took 
the curves for the 6BG6 and converted from their receiving tube format 
to constant current as use for transmitting tubes. Matched the 807/1625 
perfectly.

The color sweep tubes had large cathode surfaces for high peak currents, 
good for high class C amplifier efficiencies, but were not made for 
linear amplifier service, only the pulses of the horizontal output 
circuits. When used for linears they worked but with intermod more like 
20 or 25 dB down third order and produced lots of RF power per buck of 
tube cost. They were readily available all over while 6146 could only be 
bought from industrial or ham radio stores. They did have their 
failings, running RTTY or saying aaaaaahhh too long could melt the glass 
causing it to shrink against the inner works. Swan as well as most other 
makers sold rigs with sweep tube finals at relatively low prices.

Then bipolar PA came along. K4QF worked up intermod improvements at 
Collins but those haven't become well known to the rest of the industry. 
Bipolar and low voltage PA have tough clipping, unlike the softer 
clipping of tubes, whether audio or RF linears and so when driven hard 
have strong intermod products 3, 5, 7th, and 9th are often detected. And 
running at low power doesn't always make them cleaner. There is 
sometimes a bit of RF feedback mostly to compensate for the wild 
differences in gain between 1.8 and 30 MHz.

MOSFETs are inherently nonlinear but not as abruptly as bipolar transistors.

As solid state rigs came on the air, there were alarms from the DXers in 
urban areas noticing an increase in band noise caused by those broad 
band amplifiers. I wonder how much was transmitter and how much was in 
the observer's receiver being overloaded by the transmitter, though 
phase noise had to be a strong factor coming from imperfect synthesizers 
if both the transmitters and receivers. But generally transmitter 
intermod never bothers the user of the transmitter, just those trying to 
receive at the same time on the same band in the same neighborhood.

I've not worked in the ham gear industry, though I did work a few years 
at Collins with many of the S-line designers. I've taken the time to 
learn what makes the stuff work at the detail level and tried to improve 
on them myself. My ham station is mostly in boxes at the moment though 
it does still include an S-line, a Corsair II, and several VHF and a 
couple HF Yaesu rigs. Right now one FT-857 can be hooked to VHF/UHF 
antennas, and I was just looking at the trees needing to be adorned with 
an HF wire before snow. But first I have to finish moving out of the 
place I've rented for 40 years.

73, Jerry, K0CQ (and technical adviser to the Collins Radio Association)
>
>
> ========================= JHR =============================
>
> On 9/29/2010 1:43 AM, Dr. Gerald N. Johnson wrote:
>
>>
>> Transmitter intermod performance has gone down since the Collins S-line.
>> Started by Swan and completed by solid state PAs.
>
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